The present invention pertains to the production of chlorine dioxide gas and in particular the detection of chlorine gas in the chlorine dioxide gas product.
Chlorine dioxide is known to be a useful gas for controlling microbial contamination of a myriad of products, most importantly for disinfecting drinking water. In the treatment of water with chlorine dioxide for drinking and other uses, it is beneficial to have pure chlorine dioxide that is substantially free of molecular chlorine. Molecular chlorine can react with organic compounds in the water to produce undesirable toxic chlorinated by-products, such as trihalomethanes (THM""s), whereas pure chlorine dioxide does not react with the organic species to produce, to any significant degree, these undesirable chlorinated disinfection by-products (DBP""s).
Chlorine dioxide is also used for bleaching paper pulp. The presence of molecular chlorine in the chlorine dioxide gas can react with lignin to produce undesirable environmental pollutants, such as dioxins. Pure chlorine dioxide does not produce dioxins.
Another use for chlorine dioxide is in the sterilization of medical products. Use of chlorine dioxide for such applications is described in U.S. Pat. Nos. 4,504,422, 4,681,739 and 4,908,188. The materials from which certain medical devices are constructed are functionally unaffected by chlorine dioxide, but are incompatable with molecular chlorine. If molecular chlorine is present in the chlorine dioxide gas, some of the products being sterilized may be spoiled or rendered unsafe for use. Because chlorine dioxide has a unique chemistry it is compatible with many products that are used in health care.
For some applications of chlorine dioxide, small amounts of chlorine can be tolerated without rendering the chlorine dioxide unsuitable for the particular application.
Because chlorine dioxide can not be shipped or stored as a compressed gas it is almost always generated at the point of use. On site production of chlorine dioxide can be achieved through several processes. For example, sodium chlorate (NaClO3) can be used as the starting material and reacted with HCl according to the following reaction:
2NaClO3+4HCl greater than 2ClO2+Cl2+2NaCl+2H2O.
Sodium chlorite (NaClO2) can be used as a starting material, which is reacted with chlorine gas according to the following equation to form gaseous chlorine dioxide.
2NaClO2+Cl2 greater than 2ClO2+2NaCl.
Because chlorine is often an undesirable contaminant in chlorine dioxide, as set forth above, it is desirable to detect and measure the concentration of chlorine in the chlorine dioxide product. Because the amount of chlorine in the chlorine dioxide produced by some methods is a function of the operating parameters employed, (e.g. concentration, time, temperature, pressure), measurement of chlorine levels in the chlorine dioxide will be useful in helping to alert the operator of the generating process when chlorine in the chlorine dioxide exceeds an acceptable level.
U.S. Pat. Nos. 5,110,580, 5,234,678, 5,290,524 and 5,326,546 all describe gas-solid methods for producing chlorine dioxide gas. The principal chemical reaction utilized in these processes is that defined by the reaction of dry sodium chlorite and chlorine gas as the starting materials, the second equation noted above.
Chlorine gas, upon contact with solid sodium chlorite in a reaction cartridge, is replaced by chlorine dioxide. As the sodium chlorite is used up, the reaction front moves forward. A low, but increasing, concentration of unreacted gaseous chlorine in a high concentration of chlorine dioxide (ClO2), along with inert gaseous carrier, eventually reaches the outlet of the reaction cartridge. The level of emergent chlorine initially detectable at the outlet is called the xe2x80x9cbreakthrough concentrationxe2x80x9d. Breakthrough detection at an appropriate level signals the need to switch the chlorine-bearing input gas stream to a fresh, unreacted, sodium chlorite cartridge.
Prior to the present invention, there appears to have been no simple, effective method for detecting low breakthrough concentrations of chlorine (e.g. 500 ppm of chlorine) in the presence of, for example, 40,000 ppm of chlorine dioxide. Methods based upon inexpensive forms of mass spectrometery, for instance, would be adversely affected by the proximity and breadth of the peaks representing chlorine and chlorine dioxide. Similarly, the two molecules have overlapping ultraviolet absorbence spectra. Therefore, ultraviolet (UV) spectrophotometry is not a practical analytical technique. This is especially true when the concentration of chlorine dioxide is high relative to the concentration of chlorine. There are various methods for dissolving the gas mixture in water and measuring the concentrations in the liquid. These, under certain circumstances, may be useful for detecting the presence of chlorine, but quantitative measurements are limited in accuracy because of the imprecision of the dissolution processes. Also, solution-based processes involve multiple steps and are often difficult to automate. Therefore, there is a need to find a method for detecting and measuring chlorine gas contamination of gaseous chlorine dioxide at various levels of chlorine gas contamination.
The present invention utilizes the discovery that in the gas phase, chlorine reacts with ammonia, while chlorine dioxide does not so react. The reaction of molecular chlorine gas with ammonia in the gas phase produces a white smoke, which is believed to be ammonium chloride particles. It is further believed that the overall reaction proceeds according to the following:
xe2x80x832NH3+Cl2 greater than NH2Cl+NH4Cl.
Mixing an excess quantity of ammonia with a sample gas suspected to contain chlorine results in the density of the smoke increasing with the concentration of the chlorine gas present in the sample. According to one embodiment of the present invention the smoke can be detected by optical means, such as light absorbence, light scatter or light transmission. When a beam of light shines through a cell containing a sample of chlorine gas in chlorine dioxide gas with air and ammonia, the amount of transmitted light decreases with the amount of chlorine in the sample. According to a preferred embodiment of the present invention an effective method of detecting the presence of smoke produced by the reaction of ammonia and chlorine utilizes radiation in the red to infrared region of the spectrum (greater than 630 nanometer wavelength). It is believed that light of any wavelength that is not absorbed by chlorine, ammonia, or chlorine dioxide can be used. Other parts of the spectrum, e.g. yellow light may work.
According to the present invention, a sample of chlorine dioxide can be introduced into a transparent cell having means to emit and detect radiation of a certain frequency passed through this sample. The sample is premixed with an excess amount of ammonia so that any chlorine present in the sample will react with the ammonia to produce smoke. The smoke is then detected by a change in the light transmittance through the cell. The amount of decrease in transmittance is directly related to the quantity of chlorine present in the sample.
According to the present invention, depending upon the cycle requirements, sampling can take place periodically with the sample being drawn from the outlet of a chlorine dioxide generator. Auxiliary equipment can be used to clean the cell to avoid contaminants after each sample is removed from the cell, immediately before a new sample is introduced to the cell, or after Cl2 is detected, given that the chlorine and ammonia reaction leaves no interfering residue.
Therefore, in one aspect the present invention is a method for detecting chlorine breakthrough during production of chlorine dioxide in a chlorine dioxide generator comprising the steps of: withdrawing a sample of chlorine dioxide from a product outlet of the generator; mixing the sample of chlorine dioxide with gaseous ammonia; and introducing the mixture of chlorine dioxide and gaseous ammonia into a test receptacle wherein smoke generated by reaction of chlorine in the chlorine dioxide with ammonia can be observed.
In another aspect the present invention is a method including the step of measuring density of any smoke observed in said test receptacle.
In still another aspect, the present invention is an apparatus for detecting the presence of chlorine in a product stream from a chlorine dioxide generator comprising in combination; a test receptacle having an inlet and an outlet, the test cell fabricated from optically transparent material, means to connect the inlet to a source of the product chlorine dioxide stream and a source of gaseous ammonia or ammonia vapor, and means to detect products of reaction between the ammonia and any chlorine present in the chlorine dioxide product stream.
In yet another aspect the present invention is a process for eliminating chlorine from a gaseous stream containing chlorine dioxide, chlorine and a diluent comprising the steps of; reacting the stream containing chlorine dioxide, chlorine and a diluent with gaseous ammonia to produce a mixture of chlorine dioxide, diluent and ammonia chloride; and separating the chlorine dioxide and diluent from the ammonia chloride.